Photochromism is a reversible action of a compound which quickly changes the color when it is irradiated with light containing ultraviolet rays, such as the sunlight or the light of a mercury lamp, and resumes its initial color when it is no longer irradiated with light and is placed in a dark place. The compounds which have the above properties are called photochromic compounds and used for material of photochromic plastic lenses.
The photochromic compound used for the above application is required to be satisfying such properties that:    (1) The compound has a low coloring degree (low initial color) in a region of visible light of before being irradiated with ultraviolet rays;    (2) The compound exhibits a high coloring degree (hereinafter called color density) when it is irradiated with ultraviolet rays;    (3) The compound enables the color density to increase to a saturation at a high rate (has a high color-developing sensitivity) after it is irradiated with ultraviolet rays;    (4) The compound returns to the initial state at a high rate (has a high fading rate) after it is no longer irradiated with ultraviolet rays;    (5) Repetition of the reversible action of the compound is highly resistant; and    (6) The compound that is cured dissolves highly densely in a monomer composition that becomes a host material so as to be highly dispersed in the host material that is used.
As a photochromic compound capable of satisfying the above requirements, there has been known a chromene compound having an indeno(2,1-f)naphtho(1,2-b)pyran structure as a basic skeleton (see a pamphlet of International Laid-Open No. 99/15518 and a pamphlet of International Laid-Open No. 2001/60811).
On the other hand, it has been desired that the photochromic plastic lenses develop a color of a neutral tint such as grey or brown. The neutral tint is obtained by mixing a plurality of kinds of photochromic compounds that develop different color tones or, concretely, by mixing a compound of yellow to red color that has a maximum absorption when developing color of 430 to 530 nm and a compound of violet to blue color having a maximum absorption when developing color of 550 to 650 nm.
When the color is adjusted by the above method, however, there occur a variety of problems due to different photochromic properties of the compounds that are mixed together. For example, when a compound that develops yellow color has a recurring resistance that is smaller than that of a compound that develops blue color, there occurs a problem in that the color tone that is developed gradually changes into a bluish color tone after used for extended periods of time. Further, when a yellow compound has a color-developing sensitivity and a fading rate lower than those of a blue compound, there occurs a problem in that the color tone becomes bluish when the color is developing and the color tone becomes yellowish when the color is fading.
It is considered that the above problems can be solved by the use of a single compound that exhibits two or more absorption maxima when developing a color to develop a color of a neutral tint. As the photochromic compounds that exhibit two or more absorption maxima when developing a color, there have been known the compounds represented by the following formulas (A) to (C). However, there has not yet been known any photochromic compound that develops a color of a neutral tint while satisfying the requirements mentioned in (1) to (6) above.
That is, the chromene compound represented by the following formula (A) (see a pamphlet of International Laid-Open No. 96/14596):
has an absorption over 430 to 530 nm, which is weaker than an absorption over 550 to 650 nm, and fails to develop a color tone of a neutral tint.
A chromene compound represented by the following formula (B) (see a pamphlet of International Laid-Open No. 2000/35902):
has a high initial color and a low recurring resistance.
The chromene compound represented by the following formula (C) (see a pamphlet of International Laid-Open No. 2001/19813):
develops a color of a neutral tint but fades at a low rate and, besides, the recurring resistance is not high enough.